In synchronously operating systems, the source of timing signals is driven by a reference frequency, the source of which is commonly duplicated for reliability purposes. If the reference frequencies are not in phase with each other, upon switchover from the first reference frequency to the second reference frequency as the driving frequency, the timing signals will undergo a phase shift reflective of the difference in phase between the first and the second reference frequency. The timing signals will also undergo a temporary frequency variation as a result of the phase change. Such phase and frequency variations in the timing signals have been found to cause problems in some synchronous systems.
For example, telephone network electronic switching systems are generally redundant, for reliability purposes being comprised of two duplicate halves either one of which can carry on the system tasks in case of failure of the other half. At any one time the output signals generated by only one of the halves of the switching system are utilized as output signals of the switching system as a whole, and serve as inputs to other equipment connected to the switching system. Synchronizing reference signals which are propagated through the two halves of the switching system may be out of phase with each other by the time they arrive at the outputs of the respective halves of the switching system. As a consequence, the output signals of a source of timing signals driven by the synchronizing reference signals often undergo a phase change and a temporary frequency change when the timing signal source is disconnected from the output of the one half and connected to the output of the other half of the switching system. Such phase changes and frequency excursions have been found to cause problems, such as loss of data signals, for communication equipment that is located down-stream from the switching system and that is listening to the system's output signal stream. These problems have led CCITT--International Telegraph and Telephone Consultative Committee--to adopt a standard setting stringent limitations on time interval error, which error includes the changes in phase that the output of a telephone network switching system is allowed to undergo.
One approach of the prior art to solving the phase shift problem has been to adjust the phase of one of the reference frequency sources to equalize it with the phase of the other reference frequency source. However, this approach has not been workable in applications, such as telephony switching systems, where the cause of the phase difference has not been a single, adjustable, reference frequency source, but the cumulative effect of distributed and often unadjustable components.
A second approach taken by the prior art to alleviate the phase shift problem has been to pass one of the reference frequencies through phase delay circuitry to adjust its phase to that of the other reference frequency. With this approach, however, only discrete, highly granular changes could generally be made in the phase of the reference frequency being adjusted, due to the nature of the phase delay elements that are conventionally used for this purpose. Such granularity has often not been sufficient to meet the needs of the application, such as the needs of telephone switching systems striving to meet the stringent CCITT time interval error standards. Refinements of this technique that have sought to achieve fine granularity of phase change, to approach a continuous phase change capability, have generally been very complex and costly, inter alia requiring the use of relatively large numbers of phase delaying elements and associated manipulation circuitry.